Silicon semiconductor substrates (wafers) used in devices such as semiconductor integrated circuits have been produced from single silicon crystals based mainly on a Czochralski process (CZ process). The CZ process is a technique for growing a single crystal by immersing a seed crystal in a quartz crucible in a silicon melt, and then pulling it upwards and out of the melt. From the quartz crucible holding the silicon melt, oxygen enters into the single silicon crystal based on the CZ process. The oxygen sufficiently dissolves into the crystal when the temperature is high just after the solidification. Since the high temperature immediately proceeding crystal solidification is cooled, and the solubility of the crystal is rapidly decreased, the oxygen incorporated inside the crystal is usually in a supersaturated state.
The oxygen incorporated in the supersaturate state within the wafer obtained from the above single crystal is then precipitated as an oxide by heat-treatment in the subsequent device manufacturing step. If the precipitate (an oxide) thereof is developed at a device forming region in a surface layer of the wafer, the performance of the device may be inhibited. On the other hand, the precipitate developed in the interior of the silicon substrate is called BMD (bulk micro defect), and acts effectively as a gettering source for trapping heavy metal impurity that enters into the silicon substrate in the device manufacturing step and deteriorates the performance thereof. The gettering method using the BMD is particularly called intrinsic gettering, and widely employed as a gettering method of harmful heavy metals. The BMD is required to exist at a density of a certain degree or more in order to act effectively as a gettering source. However, the BMD existing at too high density has the disadvantage of reducing the mechanical strength of the substrate, and the like.
With regard to the above device manufacturing step, there has been proposed a heat-treating cycle of bringing the device forming region of the wafer surface layer into a denuded state, and allowing the BMD as a gettering source to be developed in the interior thereof at an effective density. Its representative technique is as follows. (a) A portion used as a no defect layer, namely a denuded layer called a denuded zone (hereinafter referred to as DZ) is formed on the surface by subjecting a wafer to oxygen out-diffusion treatment of heating for 5 to 100 hours at high temperatures of 1100° C. or above in an oxidizing atmosphere. Subsequently, (b) the wafer is heated at low temperatures of 600° C. to 750° C., thereby forming an effective precipitation nucleus in the interior of the wafer. Thereafter, (c) the wafer is heat-treated at middle temperatures of 1000° C. to 1150° C. or at high temperatures, and a BMD is grown in the precipitation nucleus so as to ensure a gettering source. The above heat treatment cycle of (a) to (c) is a treatment method called “high-low-high or high-low-middle” cycle. However, this treatment method requires much time, and its productivity is not necessarily superior.
Recently, instead of the above-mentioned heat treatment that is complicated and time-consuming, there have been proposed rapid thermal annealing (RTA) techniques capable of imparting a similar wafer depth direction structure of a BMD. These techniques can terminate a heat treatment in a very short time, namely an order of seconds, and can also suppress non-uniformity of precipitate due to variations in the thermal history and oxygen concentration of a crystal, which have caused variations in BMD density. It is known that, by carrying out a heat-treatment of growing a BMD through an Ar atmosphere RTA, in which an argon (Ar) atmosphere (hereinafter referred to as “Ar atmosphere”) is used at the time of the RTA, a DZ having a sufficient depth can be ensured in the surface layer of the wafer, enabling a high density BMD to be developed in the interior of the wafer. The form of distribution of this BMD can be called “reverse-U-shaped distribution” from its shape. There has been disclosed (for example, in Japanese Patent Unexamined Publication No. 2002-110683) that in the BMD distribution obtained from this Ar atmosphere RTA, the depth of a DZ and the density of the internal BMD can be controlled by controlling the holding temperature, holding time, and cooling rate from the holding temperature in the RTA. With the technique disclosed in this publication, however, the BMD density in a region near a device forming region in a wafer surface layer is not high, and a region where a BMD serving as a gettering source of a harmful heavy metal exists at a high density is present apart from the surface layer. It is therefore difficult to obtain proximity gettering effect that is highly desired by device manufacturers in the recent years. Specifically, because the tendency of low temperatures in a device process reduces the diffusion rate of a heavy metal that has contaminated a device forming region, the device manufacturers desire to form a BMD serving as a gettering source in a region as close as possible to the device forming region in the surface layer.
A silicon wafer having an ideal BMD distribution to meet the above desire is a silicon wafer in which a denuded layer having a sufficient depth is formed in its surface, and a BMD serving as a gettering source is formed at a high density in a position near its surface layer. In addition, it is desired that the density of the internal BMD is not too high, because too high BMD density in the interior of the wafer may cause the drawbacks such as a lowering of the mechanical strength of the substrate and the like, as described above. As contrasted to the reverse-U-shaped distribution in the Ar atmosphere RTA, such an ideal BMD distribution can be called an M-shaped distribution. The M-shaped distribution of the BMD can be obtained by an RTA in an atmosphere of nitriding gas such as N2, NH3 or the like, or in a mixed gas atmosphere of these nitriding gas, Ar (argon), O2 (oxygen), H2 (hydrogen), and the like. For example, after an RTA of rapid heating and cooling (for example, temperature increase or decrease of 50° C./sec) under an RTA holding temperature in a range of 800° C. to 1280° C., and an RTA holding time of from one second to five minutes, oxygen precipitation heat-treatment (for example, four hours at 800° C. plus 16 hours at 1000° C.) is carried out, so that a DZ is formed in the surface layer of a wafer, and high-density layers having the maximum BMD density are formed in the interior proximate to the DZ, and furthermore, low-density layers having the minimum BMD density are formed in the inside of these high-density layers (for example, refer to Japanese Patent Unexamined Publication No. 2003-7711). This publication describes that the above-mentioned maximum BMD density is set to 3.5×109 cm−3 or more, and the above-mentioned minimum BMD density is set to 2.5×108 cm−3 or less. Subsequently, a substrate material whose oxygen concentration is 11×1017 atoms/cm3 to 17×1017 atoms/cm3 is used and heated to 1100° C. to 1300° C. in a nitrogen-containing atmosphere by setting a rate of increase in temperature to 10 to 30° C./sec, and then an RTA process is carried out at a cooling rate of 1 to 25° C./sec, thereby enabling to manufacture a silicon semiconductor substrate in which the wafer surface is provided with a denuded layer having a depth of 10 μm or more, and the mid-portion in the wafer depth has a low BMD density and a region near the denuded layer in the surface has a high BMD density (for example, refer to the above-mentioned Japanese Patent Unexamined Publication No. 2002-110683 or 2003-7711).
Nevertheless, these publications disclose clearly neither the form of the M-shaped distribution of the BMD, that is, the maximum value of the BMD density and the distance from the wafer surface in the position indicating the value, nor a specific method of controlling the BMD density in the interior of the wafer to an arbitrary value. This seems to be because the most important factor in determining the M-shaped distribution of the BMD cannot be controlled accurately. Thus, these conventional techniques do not describe the method of controlling the BMD distribution to an arbitrary shape. It is therefore difficult for these techniques to individually meet the demands for proximate gettering functions of silicon wafers which can vary from manufacturer to manufacturer.
It is an object of the present invention to provide a silicon wafer in which, in an M-shaped distribution of BMD density in a depth direction of a wafer obtained by rapid thermal annealing in a nitrogen-containing atmosphere, the distribution form thereof is arbitrarily controlled, and also provide heat-treatment method for stably obtaining this silicon wafer.